Mice homozygous for a gene (iv) causing situs inversus will be used as a model for human syndromes characterized by congenital heart disease and heterotaxia. These human syndromes are: 1) asplenia-polysplenia, 2) interruption of inferior vena cava with azygos-inferior vena cava continuity, 3) isolated dextro- and levocardia, and 4) preduodenal portal vein. All of these human syndromes have similar, often overlapping, patterns of heterotaxia and have similar kinds of heart malformations. The iv/iv mice have these same kinds of heterotaxia and heart malformations. They will be used to study early stages of development of heart malformations including: common persistant atrio-ventricular canal, transposition of the great arteries, double outlet right ventricle, anomalous pulmonic and systemic venous return and atrial isomerism. These studies will use the techniques of scanning and transmission electron microscopy, reconstruction of serial sections and organ culture. In addition, studies will be done on the developmental biology and genetics of situs inversus.